The following article will guide you about how to control noise pollution from diesel generator set.
Introduction:
In recent years the shortage of power coupled with unreliable and poor quality of the power supply has resulted in a proliferation of captive generator sets, run with diesel, petrol or kerosine, as an alternate source of power. This is happening, particularly, in urban areas where the need is greater.
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Unfortunately, in most of these places there is an overlap of commercial, residential and industrial areas which together with poor planning for the installation of generator sets and absence of any acoustic treatment/enclosure, leads to excessive noise pollution. An effort has been made, here, to tackle this problem through prescribing noise standards and developing guidelines for providing acoustic enclosure/ treatment for diesel run generator (DG) sets.
The study includes the noise analysis of DG sets covering the power range of 15-500 KVA (1500 rpm), prescribing noise standards at the manufacturing stage and at the user’s end, design of acoustic enclosure and developing guidelines for control of noise pollution from DG sets.
Noise analysis for a number of DG sets of various makes and types, covering the power range of 15 to 500 KVA and rpm of 1500 have been carried out for the purpose of the study. Based on these measurements, noise standards for DG sets at the manufacturing stage and at the user’s end have been recommended. In addition, guidelines have been prescribed for both, the manufacturers and the users, to control noise pollution by providing acoustic enclosure/treatment and by proper installation, maintenance and other means.
In a typical DG set, the primary sources of noise are – the engine body, air intake, exhaust, the cooling fan and other moving components. Incidentally, the alternator is relatively much quieter.
Sound pressure levels in the hemispherical free field at 1 metre distance were calculated. The calculations were based on actual measurements and plotted against nominal power rating of the DG set in KVA (drawn on a logarithmic scale).
A least square fit line is drawn representing a typical average variation for all water-cooled engines, which is given by the following symbolic relationship –
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LPA (1 metre, hemispherical free field) = 91.8 + 3 log10 (KVA), dBA, (For water-cooled engines)
The corresponding relationship for sound power level is given by –
LWA = 99.8 + 3 log10 (KVA), dBA, (For water-cooled engines)
The corresponding level (Pounds power level) for air-cooled engines is about 5 dB lower and is given by the following relationship –
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Lwa = 94.8 + 3 logl0 (KVA), dBA, (For air-cooled engines).
Modification of each component is required to reduce its vibration response. For sheet metal components, damping can be used effectively to control the response. For thicker components, an isolation system will be an effective solution.
Exhaust noise is by far the most significant component of the noise emanating from the engine. However, it can be reduced to the level of the total noise of all other components or even lower, by means of a reflective muffler.
While exhaust noise can be taken care of by a well-designed acoustic muffler, all other sources of noise can be controlled by a well acoustic enclosure.
Identification of Noise Sources:
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In a typical DG set, the primary sources of noise are:
1. The cooling fan.
2. The exhaust.
3. The engine body noise.
4. The intake.
5. Other moving components.
The effect of different loads on the noise level is much weaker than the effect of speed. Of interest here is the fact that a number of components on diesel engine exhibit similar overall noise levels. Incidentally, the alternator is relatively much quieter.
Noise Analysis of DG Sets:
Bangalore is a cosmopolitan city with well-developed industrial environment and a large number of establishments and companies deploying DG sets. Thus it was possible to locate a sufficient number of DG sets of various makes and types in and around Bangalore covering the power range of 15 KVA to 500 KVA and rpm of 1500. Details of these installations are given in Table 11.1.
Methods of Noise Reduction/Control:
Modification of the response of each component is required to reduce its vibration response. For sheet metal components like oil pan, damping can be used effectively to control the response. For thicker components, an isolation system will be an effective solution. The best known and most widely used method of reduction of fan noise is reduction of fan tip speed.
The fan noise can also be reduced by optimising:
1. The radiator design in case of water-cooled engines.
2. The fan shroud design.
3. The axial position of the fan with respect to the shroud.
Exhaust noise is by far the most significant component of engine noise. Fortunately, it can be reduced to the level of the total noise of the all other components or even lower, by means of a reflective muffler.
While exhaust noise is taken care of by a well-designed exhaust muffler, all other sources of noise can be reduced by a well-designed acoustic enclosure. Intake noise is inherently lower than exhaust noise. An air cleaner or filter provides some attenuation. If needed, an additional muffler (generally of the resonator type) can be provided.
Taking into account the unavoidable acoustic leaks in the fabrication of acoustic enclosures, an insertion loss (IL) of 25 dBA is achievable with normal resources and caution. For an additional 5 dBA IL or noise reduction, extreme care will have to be taken in the fabrication of the acoustic enclosure.
Here, IL is defined as the difference in sound levels at a particular point (say at 1 metre distance) with and without the acoustic enclosure. This is the real criterion of the acoustic performance and should form the basis of all testing of acoustic enclosures.
For upto 30 dBA IL, a 16 gauge (1.6 mm thick) mild steel plate would suffice for fabrication of the acoustic enclosure. This could be in the form of a modular box type structure which would provide sufficient stiffness and the necessary rigidity. This will have to be lined on the inside with an absorptive layer, typically a 10 cm thick layer of glass wool or mineral wool.
Glass wool of about 24 kg/m3 density or more or mineral wool of 48 kg/m3 density or more will be adequate with the exposed surface being retained by a thin, highly perforated layer of aluminium or galvanised steel. Mineral wool has a tendency to settle down, producing gaps or pockets with time due to excessive vibration, unless proper grade material with longer strands and proper bonding as required for acoustic applications is provided.
Air required for the ventilation and breathing of the engine will have to be provided by means of intake louvres and exhaust louvres (called parallel baffle mufflers) projecting out of the enclosure. In Fig. 11.5, the X-axis represent the non-dimensionalised frequency parameter and the Y-axis represents the transmission loss for a length equal to h, half the passage width (refer insert in Fig. 11.5). The transmission loss across length 1 of the muffler will be given by-
TL1 = TLh × 1/h … (11.7)
It is suggested that d = h = 50 mm (refer Fig. 11.5). Length 1 should be 1 metre for 25 dB IL and 1.25 metre for 30 dB IL.
The cross dimension W and the number of passes for parallel baffle sets ‘n’ will be decide by the following formula-
‘n’ (2h + 2d) W > 1.5 (π/4 x D2) … (11.8)
Where D is the diameter of the cooling fan of the engine. Thickness of the plate used in the louver or baffles.
It is worth repeating here that utmost care should be taken to avoid all acoustic leaks. All metal to metal joints should be through the agency of thick, soft, gasket of foam rubber etc. The DG set should rest on well-designed vibration mounts. The enclosure should not be resting on the floor directly; thick rubber mats should be provided in order to avoid structure borne sound.
The enclosure should be designed for easy accessibility to facilitate operational and routine maintenance. The access should be through an acoustically sealed door frame with a latch to keep it fully closed during DG set operation, pressing against thick, soft rubber gaskets.
All connections for water, fuel and electricity should be vibration isolated in order to avoid flanking transmission.
Proposed Standards/Guidelines for Control of Noise Pollution from Stationary Diesel Generator Sets:
Noise Standards for DG Sets (15-500 KVA) (Water-cooled):
The sound power level, LWA, of a DG set should be less than-
94 + 10 log10 (KVA), dBA, at the manufacturing stage.
(KVA is the nominal power rating of a DG set)
These levels should fall by 5 dBA every five years, till 2007, i.e., in 2002 and then in 2007.
Noise from the DG set should be controlled by providing an acoustic enclosure or by treating the room acoustically.
The acoustic enclosure/acoustic treatment of the room should be designed for a minimum insertion loss of 25 dBA or for meeting the ambient noise standards, whichever is on the higher side (If the actual ambient noise is on the higher side, it may not be possible to check the performance of the acoustic enclosure/acoustic treatment.
Under such circumstances the performance may be checked for noise reduction upto an actual ambient noise level, preferably, during the night time). The measurement for insertion loss may be done at different points at 0.5 metre from the acoustic enclosure/room and then averaged. The DG set should also be provided with a proper exhaust muffler with insertion loss of minimum 25 dBA.
Guidelines for the Manufacturers/Users of DG Sets (5 KVA and above) (Both Water and Air-Cooled):
1. The manufacturer should offer to the user a standard acoustic enclosure of 25 dBA insertion loss. Similarly, the manufacturer should offer a suitable exhaust muffler with an insertion loss of 25 dBA to the user.
2. The user should make efforts to bring down the noise levels due to the DG set, outside his premises, within the ambient noise requirements.
To achieve this, he may adopt one or more of the following measures:
(a) Locate the DG set at a proper site.
(b) Make use of a standard acoustic enclosure or get the DG set room treated acoustically so that the DG set room itself acts as an acoustic enclosure.
(c) Lead the exhaust gases away through a suitable exhaust muffler.
3. The customer should insist on DG set manufacturers to furnish noise power levels of the DG sets as per standards prescribed.
4. If required, the customer should insist on the manufacturers to provide a standard acoustic enclosure of minimum 25 dBA insertion loss.
5. The sound power level of a DG set, at the user’s end, shall be within 2 dBA of the sound power level of the DG set, at the manufacturing stage.
6. Installation of a DG set must be strictly in compliance with recommendations of the DG set manufacturer. This would ensure an installation free from vibration and exhaust gas leaks which are also major contributors to increased noise levels.
7. A proper routine and preventive maintenance procedure for the DG set should be set and followed in consultation with the DG set manufacturer which would help prevent noise levels of the DG set from deteriorating with use.
In this regard, some of the important aspects are:
(a) Minimise vibration levels and maintain it at all times within the limits prescribed by the DG set manufacturer. To ensure this, have engine-alternator alignment checked once every six months and similarly check tightness of all belts, bolts and other clamping devices.
(b) Anti-vibration mounts on which the DG set is mounted, generally, have rubber elements which are prone to deterioration when they come in contact with oil and diesel fuel. Hence, they should be kept clean and replaced whenever necessary.
(c) The exhaust piping from the engine exhaust main should be checked periodically for leaks and accordingly plugged.
(d) Exhaust silencers also tend to deteriorate with use and will need replacement when performance degrades.
Note:
In the document, it is reported that sound power level is related to the log of KVA, nominal power rating of the DG set, by a factor of 3. Whereas, in the proposed standard it is related to the log of KVA by a factor of 10. This has been done to bring uniformity with the international standards. This would require measurement of sound pressure level at 1 metre from the centre line of the DG set.